Illumination apparatus

ABSTRACT

An illumination apparatus, comprising at least one light emitting source embedded in a waveguide material is disclosed. The waveguide material is capable of propagating light generated by light emitting source(s), such that at least a portion of the light is diffused within the waveguide material and exits through at least a portion of its surface.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/289,442, filed on Nov. 4, 2011, which is a continuation of U.S.patent application Ser. No. 11/921,305, filed on Nov. 30, 2007, which isthe U.S. national stage application of International (PCT) PatentApplication Serial No. PCT/IL2006/000667, filed Jun. 7, 2006, whichclaims the benefit of U.S. Provisional Patent Application No.60/687,865, filed Jun. 7, 2005. The entire disclosure of each of theseapplications is incorporated by reference herein.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to artificial illumination and, moreparticularly, but not exclusively to an illumination apparatus capableof providing light at any intensity profile and any color profile,including, without limitation, uniform white light.

Artificial light can be generated in many ways, including,electroluminescent illumination (e.g., light emitting diodes),incandescent illumination (e.g., conventional incandescent lamps,thermal light sources) and gas discharge illumination (e.g., fluorescentlamps, xenon lamps, hollow cathode lamps). Light can also be emitted viadirect chemical radiation discharge of a photoluminescent (e.g.,chemoluminescence, fluorescence, phosphorescence).

A light emitting diode (LED) is essentially a p-n junction semiconductordiode that emits a monochromatic light when operated in a forward biaseddirection. In the diode, current flows easily from the p-side to then-side but not in the reverse direction. When two complementarycharge-carriers (an electron and a “hole”) collide, the electron-holesystem experiences a transition to a lower energy level and emits aphoton. The wavelength of the light emitted depends on the deferencebetween the two energy levels, which in turn depends on the band gapenergy of the materials forming the p-n junction.

LEDs are used in various applications, including, traffic signal lamps,large-sized full-color outdoor displays, various lamps for automobiles,solid-state lighting devices, flat panel displays and the like. Thebasic structure of a LED consists of the light emitting semiconductormaterial, also known as the bare die, and numerous additional componentsdeigned for improving the performance of the LED. These componentsinclude a light reflecting cup mounted below the bare die, a transparentencapsulation, typically silicone, surrounding and protecting the baredie and the light reflecting cup, and bonders, for supplying theelectrical current to the bare die. The bare die and the additionalcomponents are efficiently packed in a LED package.

Nowadays, the LED has won remarkable attention as a next-generationsmall-sized light emitting source. The LED has heretofore had advantagessuch as a small size, high resistance and long life, but has mainly beenused as indicator illumination for various measuring meters or aconfirmation lamp in a control state because of restrictions on a lightemitting efficiency and light emitting output. However, in recent years,the light emitting efficiency has rapidly been improved, and it is saidto be a matter of time that the light emitting efficiency exceeds thatof a high-pressure mercury lamp or a fluorescent lamp of a dischargetype which has heretofore been assumed to have a high efficiency. Due tothe appearance of the high-efficiency high-luminance LED, a high-outputlight emitting source using the LED has rapidly assumed apracticability. In recent years, a blue LED has been brought into apractical use stage in addition to conventional red and green LEDs, andthis has also accelerated the application of the LED.

The application of the high-efficiency high-luminance LED has beenconsidered as a promising small-sized light emitting source of anilluminating unit which is requested to have a light condensingcapability. The LED originally has characteristics superior to those ofanother light emitting source, such as life, durability, lighting speed,and lighting driving circuit. Furthermore, above all, blue is added, andthree primary colors are all used in a self-light emitting source, andthis has enlarged an application range of a full-color image displays.

Luminescence is a phenomenon in which energy is absorbed by a substance,commonly called a luminescent, and emitted in the form of light. Theabsorbed energy can be in a form of light (photons), electrical field orcolliding particles (e.g., electrons). The wavelength of the emittedlight differs from the characteristic wavelength of the absorbed energy(the characteristic wavelength equals hc/E, where h is the Plank'sconstant, c is the speed of light and E is the energy absorbed by theluminescent).

The luminescence is a widely occurring phenomenon which can beclassified according to the excitation mechanism as well as according tothe emission mechanism. Examples of such classifications includephotoluminescence, electroluminescence, fluorescence andphosphorescence. Similarly, luminescent materials are classified intophotoluminescents materials, electroluminescent materials, fluorescentmaterials and phosphorescent materials, respectively.

A photoluminescent is a material which absorbs energy is in the form oflight, an electroluminescent is a material which absorbs energy is inthe form of electrical field, a fluorescent material is a material whichemits light upon return to the base state from a singlet excitation, anda phosphorescent materials is a material which emits light upon returnto the base state from a triplet excitation.

In fluorescent materials, or fluorophores, the electron de-excitationoccurs almost spontaneously, and the emission ceases when the sourcewhich provides the exciting energy to the fluorophore is removed.

In phosphor materials, or phosphors, the excitation state involves achange of spin state which decays only slowly. In phosphorescence, lightemitted by an atom or molecule persists after the exciting source isremoved.

Luminescent materials are selected according to their absorption andemission characteristics and are widely used in cathode ray tubes,fluorescent lamps, X-ray screens, neutron detectors, particlescintillators, ultraviolet (UV) lamps, flat panel displays and the like.

Luminescent materials, particularly phosphors, are also used foraltering the color of LEDs. Since blue light has a short wavelength(compared, e.g., to green or red light), and since the light emitted bythe phosphor has a longer wavelength than the absorbed light, blue lightgenerated by a blue LED can be readily converted to produce visiblelight having a longer wavelength. For example, a blue LED coated by asuitable yellow phosphor can emit white light. The phosphor absorbs thelight from the blue LED and emits in a broad spectrum, with a peak inthe yellow region. The photons emitted by the phosphor and thenon-absorbed photons emitted of the LED are perceived together by thehuman eye as white light. The first commercially available phosphorbased white led was produced by Nichia Co. The white LED consisted of agallium indium nitride (InGaN) blue LED coated by a yellow phosphor.

In order to get sufficient brightness, a high intensity LED is needed toexcite the phosphor to emit the desired color. As commonly known whitelight is composed of various colors of the whole range of visibleelectromagnetic spectrum. In the case of LEDs, only the appropriatemixture of complementary monochromatic colors can cast white light. Thisis achieved by having at least two complementary light sources in theproper power ratio. A “fuller” light (similar to sunlight) can beachieved by adding more colors. Phosphors are usually made of zincsulfide or yttrium oxides doped with certain transition metals (Ag, Mn,Zn, etc.) or rare earth metals (Ce, Eu, Tb, etc.) to obtain the desiredcolors.

In a similar mechanism, white LEDs can also be manufactured usingfluorescent semiconductor material instead of a phosphor. Thefluorescent semiconductor material serves as a secondary emitting layer,which absorbs the light created by the light emitting semiconductor andreemits yellow light. The fluorescent semiconductor material, typicallyan aluminum gallium indium phosphide (AlGaInP), is bonded to the primarysource wafer.

Another type of light emitting device is an organic light emitting diode(OLED) which makes use of thin organic films. An OLED device typicallyincludes an anode layer, a cathode layer, and an organic light emittinglayer containing an organic compound that provides luminescence when anelectric field is applied. OLED devices are generally (but not always)intended to emit light through at least one of the electrodes, and mayinclude one or more transparent electrodes.

Combination of LEDs, OLEDs and luminescence is widely used in the fieldof electronic display devices.

Many efforts have been made to research and develop various electronicdisplay devices. Electronic display devices may be categorized intoactive display devices and passive display devices. The active displaydevices include the cathode ray tube (CRT), the plasma display panel(PDP) and the electroluminescent display (ELD). The passive displaydevices include liquid crystal display (LCD), the electrochemicaldisplay (ECD) and the electrophoretic image display (EPID).

In active display devices, each pixel radiates light independently.Passive display devices, on the other hand, do not produce light withinthe pixel and the pixel is only able to block light. In LCD devices, forexample, an electric field is applied to liquid crystal molecules, andan alignment of the liquid crystal molecule is changed depending on theelectric field, to thereby change optical properties of the liquidcrystal, such as double refraction, optical rotatory power, dichroism,light scattering, etc. Since LCD are passive, they display images byreflecting external light transmitted through an LCD panel or by usingthe light emitted from a light source, e.g., a backlight assembly,disposed below the LCD panel.

An LCD includes a LCD panel and backlight assembly. The LCD panelincludes an arrangement of pixels, which are typically formed of thinfilm transistors fabricated on a transparent substrate coated by aliquid crystal film. The pixels include three color filters, whichtransmit one third of the light produced by each pixel. Thus, each LCDpixels is composed of three sub-pixels. The thin film transistors areaddressed by gate lines to perform display operation by way of thesignals applied thereto through display signal lines. The signals chargethe liquid crystal film in the vicinity of the respective thin filmtransistors to effect a local change in optical properties of the liquidcrystal film.

Typical backlight assembly of LCD includes an array of white LEDs foremitting white light, a light guiding plate for guiding the light towardthe LCD panel, a reflector, disposed under the light guiding plate toreflect the lights leaked from the light guiding plate toward the lightguiding plate, and optical sheets for enhancing brightness of the lightexited from the light guiding plate. Backlight assembly are designed toachieve many goals, including high brightness, large area coverage,uniform luminance throughout the illuminated area, controlled viewingangle, small thickness, low weight, low power consumption and low cost.

In operation, the backlight assembly produces white illuminationdirected toward the LCD pixels. The optical properties of the liquidcrystal film are locally modulated by the thin film transistors tocreate a light intensity modulation across the area of the display. Thecolor filters colorize the intensity-modulated light emitted by thepixels to produce a color output. By selective opacity modulation ofneighboring pixels of the three color components, selected intensitiesof the three component colors are blended together to selectivelycontrol color light output. Selective the blending of three primarycolors such as red, green, and blue (RGB) can generally produce a fullrange of colors suitable for color display purposes.

LCD devices are currently employed in many applications (cellularphones, personal acceptance devices, desktop monitors, portablecomputers, television displays, etc.), and there is a growing attentionto devise backlight high-quality assemblies for improving the imagequality inn these applications.

Since the back light must pass through the color filters it thereforemust include the wavelength at which the respective filter istransparent. However, the use of white LED composed of a blue LED coatedby a yellow phosphor, is not efficient for backlighting because,although such dichromatic light appears as white light for the humaneye, it cannot efficiently pass through the RGB color filters. Anotherapproach is to use red green and blue LEDs which match the centralwavelength of each color filter. This approach significantly complicatesthe manufacturing process because the red, green and blue LEDs have tobe accurately aligned in a multichip approach. An additional approach isto generate white light using a UV LED and three different phosphorseach emitting light at a different wavelength (red, green and blue). Theefficiency of this configuration, however, is very low because highamount of heat is released due to the Stokes shift. Other configurationsinclude, two LEDs and a phosphor (e.g., a blue LED a red LED and a greenphosphor), three LEDs and a phosphor (e.g., a blue LED, a red LED, acyan LED and a green phosphor), two LEDs and two phosphors (e.g., a blueLED, a red LED, a cyan phosphor and a green phosphor), and the like.Although these configurations show improvement in the performances, theresults are far from being optimal.

Presently known LED based backlight devices are limited by the size,price and performance of the LEDs. To date, the performance of the LEDare controlled by its transparent encapsulation which provides thenecessary light scatter, the phosphor or fluorescent semiconductormaterial which is responsible for color conversion, and the lead framewhich allows for heat evacuation, all of which significantly increasethe size and cost of the LED. Since the performance, cost and size ofthe LED are conflicting features, some compromises are inevitable.

There is thus a widely recognized need for, and it would be highlyadvantageous to have a diode-based illumination apparatus, devoid of theabove limitations.

SUMMARY OF THE INVENTION

The background art does not teach or suggest illumination apparatuscapable of generating light while, at the same time, propagating,diffusing and emitting the light through its surface area or a portionthereof. The background art also does not teach or suggest suchillumination apparatus having superior optical and/or geometricalproperties, including, without limitation, a thickness of a fewcentimeters or less, and high light quality in terms of brightness,intensity and/or color profiles. The background art also does not teachsuch illumination apparatus that is useful for a variety of differentapplications, e.g., passive displays, which can benefit from itssuperior properties.

The present embodiments overcome these deficiencies of the backgroundart by providing an illumination apparatus, which generates and diffuseslight. The illumination apparatus comprises one or more light emittingsources embedded in a waveguide material. The waveguide material iscapable of propagating light generated by the light emitting source(s),such that at least a portion of the light is diffused within thewaveguide material and exits through at least a portion of its surface.In various exemplary embodiments of the invention the light emittingsource(s) comprise bare dies.

The apparatus of the present embodiments can be incorporated in apassive display device or it can serve for providing signage or forproviding illumination at various decorative patterns of significantaesthetic interest. In various exemplary embodiments of the inventionthe apparatus serves as a component in a liquid crystal display device.

According to another aspect of the present invention there is provided amethod of providing illumination, comprising applying forward bias onone or more light emitting sources embedded in a waveguide material toeffect emission of light from the light emitting source(s). Thewaveguide material is capable of propagating the light such that atleast a portion of the light is diffused within the waveguide materialand exits through at least a portion of its surface, thereby providingthe illumination.

According to yet another aspect of the present invention there isprovided a liquid crystal display device, comprising a liquid crystalpanel and a backlight unit, which preferably comprises the illuminationapparatus described herein.

The present embodiments further comprise a portable computer systemcomprising the liquid crystal display device, a computer monitorcomprising the liquid crystal display device, a personal digitalassistant system comprising the liquid crystal display device, acellular communication system comprising the liquid crystal displaydevice, and a television system comprising the liquid crystal displaydevice.

The present embodiments further comprise a signage panel for producing asignage under electronic control. The signage panel comprises a powersource connected to the illumination apparatus described herein.

According to further features in preferred embodiments of the inventiondescribed below, the waveguide material is flexible.

According to still further features in the described preferredembodiments the light emitting source(s) comprises at least one bare dieand electrical contacts connected thereto.

According to still further features in the described preferredembodiments the light emitting source(s) is encapsulated by atransparent thermal isolating encapsulation.

According to still further features in the described preferredembodiments the light emitting source(s) is embedded near the secondsurface of the waveguide material.

According to still further features in the described preferredembodiments the light emitting source(s) is embedded near the secondsurface in a manner such that electrical contacts of the light emittingsource(s) remain outside the waveguide material at the second surface.

According to still further features in the described preferredembodiments the illumination apparatus further comprises a printedcircuit board electrically connected to the electrical contacts.

According to still further features in the described preferredembodiments the printed circuit board is capable of evacuating heat awayfrom the light emitting source(s).

According to still further features in the described preferredembodiments the light emitting source(s) is embedded in the bulk of thewaveguide material.

According to still further features in the described preferredembodiments the illumination apparatus further comprises an arrangementof electrical lines extending from the light emitting source(s) to atleast one end of the waveguide material.

According to still further features in the described preferredembodiments the light emitting source(s) comprises at least one sourceconfigured to emit light at a first color, and at least one sourceconfigured to emit light at a second color.

According to still further features in the described preferredembodiments the liquid crystal panel comprises an arrangement of colorfilters operating at a plurality of distinct colors, and the lightemitting source(s) comprises, for each color of the plurality ofdistinct colors, at least one source configured to emit light at thecolor.

According to still further features in the described preferredembodiments the illumination apparatus further comprises a reflectingsurface embedded in or attached to the waveguide material in a mannersuch that emission of light through the second surface is prevented andemission of light through the first surface is enhanced.

According to still further features in the described preferredembodiments the illumination apparatus further comprises at least onephotoluminescent layer coating at least the portion of the firstsurface.

According to still further features in the described preferredembodiments the at least one photoluminescent layer and the lightemitting source(s) are selected such the illumination apparatus providessubstantially white light.

According to still further features in the described preferredembodiments the liquid crystal panel comprises an arrangement of colorfilters operating at a plurality of distinct colors, wherein the atleast one photoluminescent layer and the light emitting source(s) areselected such the illumination apparatus provides light at least at thea plurality of distinct colors.

According to still further features in the described preferredembodiments the at least one photoluminescent layer comprises aplurality of photoluminescent layers, each being characterized by adifferent absorption spectrum, and the light emitting source(s)comprises, for each absorption spectrum, at least one light emittingsource characterized by an emission spectrum overlapping the absorptionspectrum.

According to still further features in the described preferredembodiments the illumination apparatus further comprises a structuredfilm deposited on or embedded in the waveguide material.

According to still further features in the described preferredembodiments the illumination apparatus further comprises at least oneoptical element embedded in the waveguide material for enhancingdiffusion of light within the waveguide material.

According to still further features in the described preferredembodiments the at least one optical element is embedded in thewaveguide material near the first surface and configured to reflectlight striking the at least one optical element at a predetermined rangeof angles, and transmit light striking the at least one optical elementat other angles.

According to still further features in the described preferredembodiments the at least one optical element comprises at least onemicrolens.

According to still further features in the described preferredembodiments the waveguide material comprises a polymeric material.

According to still further features in the described preferredembodiments the waveguide material comprises a rubbery material.

According to still further features in the described preferredembodiments the waveguide material is formed by dip-molding in a dippingmedium.

According to still further features in the described preferredembodiments the dipping medium comprises a hydrocarbon solvent in whicha rubbery material is dissolved or dispersed.

According to still further features in the described preferredembodiments the dipping medium comprises additives selected from thegroup consisting of cure accelerators, sensitizers, activators,emulsifying agents, cross-linking agents, plasticizers, antioxidants andreinforcing agents

According to still further features in the described preferredembodiments the waveguide material comprises a dielectric material, andwherein a reflection coefficient of the dielectric material is selectedso as to allow propagation of polarized light through the waveguidematerial and emission of the polarized light through the first surface.

According to still further features in the described preferredembodiments the waveguide material comprises a metallic material, andwherein a reflection coefficient of the metallic material is selected soas to allow propagation of polarized light through the waveguidematerial and emission of the polarized light through the first surface.

According to still further features in the described preferredembodiments the waveguide material is a multilayered material.

According to still further features in the described preferredembodiments the waveguide material comprises a first layer having afirst refractive index, and a second layer being in contact with thefirst layer and having a second refractive index being larger that thefirst refractive index.

According to still further features in the described preferredembodiments the second layer comprises polyisoprene.

According to still further features in the described preferredembodiments the first layer comprises silicone.

According to still further features in the described preferredembodiments the waveguide material further comprises a third layer beingin contact with the second layer and having a third refractive indexbeing smaller than the second refractive index.

According to still further features in the described preferredembodiments the third refractive index equals the first refractiveindex.

According to still further features in the described preferredembodiments at least one layer of waveguide material comprises at leastone additional component designed and configured such as to allow theemission of the light through the at least a portion of the surface.

According to still further features in the described preferredembodiments the at least one additional component is capable ofproducing different optical responses to different spectra of the light.

According to still further features in the described preferredembodiments the different optical responses comprise different emissionangles.

According to still further features in the described preferredembodiments the different optical responses comprise different emissionspectra.

According to still further features in the described preferredembodiments the at least one additional component comprises at least oneimpurity capable of emitting at least the portion of the light throughthe first surface.

According to still further features in the described preferredembodiments the at least one impurity comprises a plurality of particlescapable of scattering the light.

According to still further features in the described preferredembodiments a size of the plurality of particles is selected so as toselectively scatter a predetermined spectrum of the light.

The present invention successfully addresses the shortcomings of thepresently known configurations by providing an illumination apparatusand various uses thereof.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. In case of conflict, the patentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawings:

FIGS. 1 a-b are schematic illustrations of a perspective view (FIG. 1 a)and a section view (FIG. 1 b) of an illumination apparatus, according tovarious exemplary embodiments of the present invention;

FIG. 1 c is a schematic illustration of a perspective view of theapparatus in a preferred embodiment in which the apparatus comprises anon-planar waveguide material;

FIGS. 2 a-b are schematic fragmentary views of the apparatus in apreferred embodiment in which a source or sources embedded in the bulkof the waveguide material (FIG. 1 a), and in another preferredembodiment in which the source or sources are embedded near the surfaceof the waveguide material (FIG. 1 b);

FIG. 3 is a schematic illustration of a section view of the apparatus,in a preferred embodiment in which the apparatus comprises a structuredfilm;

FIG. 4 is a schematic illustration of a fragmentary view of theapparatus in a preferred embodiment in which the apparatus comprises oneor more embedded optical elements for enhancing the diffusion of light;

FIG. 5 is a block diagram schematically illustrating a liquid crystaldisplay device, according to various exemplary embodiments of thepresent invention;

FIG. 6 a is a schematic illustration of the waveguide material in apreferred embodiment in which two layers are employed;

FIGS. 6 b-c are schematic illustrations of the waveguide material inpreferred embodiments in which three layers are employed;

FIG. 7 a is a schematic illustration of the waveguide material in apreferred embodiment in which at least one impurity is used forscattering light;

FIG. 7 b is a schematic illustration of the waveguide material in apreferred embodiment in which the impurity comprises a plurality ofparticles having a gradually increasing concentration;

FIG. 7 c is a schematic illustration of the waveguide material in apreferred embodiment in which one layer thereof is formed with one ormore diffractive optical elements for at least partially diffracting thelight; and

FIG. 7 d is a schematic illustration of the waveguide material in apreferred embodiment in which one or more regions have different indicesof refraction so as to prevent the light from being reflected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments comprise an apparatus, device and system whichcan be used for providing illumination or displaying images.Specifically, the embodiments can be used to provide light at anyintensity profile and any color profile. The present embodiments areuseful in many areas in which illumination is required, including,without limitation, display, signage and decoration applications.

The principles and operation of an apparatus, device and systemaccording to the present invention may be better understood withreference to the drawings and accompanying descriptions.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

When a ray of light moves within a transparent substrate and strikes oneof its internal surfaces at a certain angle, the ray of light can beeither reflected from the surface or refracted out of the surface intothe open air in contact with the substrate. The condition according towhich the light is reflected or refracted is determined by Snell's law,which is a mathematical relation between the impinging angle, therefracting angle (in case in case of refraction) and the refractiveindices of both the substrate and the air. Broadly speaking, dependingon the wavelength of the light, for a sufficiently large impinging angle(also known as the critical angle) no refraction can occur and theenergy of the light is trapped within the substrate. In other words, thelight is reflected from the internal surface as if from a mirror. Underthese conditions, total internal reflection is said to take place.

Many optical systems operate according to the total internal reflectionphenomenon. One such optical system is the optical fiber. Optical fibersare transparent flexible rods of glass or plastic, basically composed ofa core and cladding. The core is the inner part of the fiber, throughwhich light is guided, while the cladding surrounds it completely. Therefractive index of the core is higher than that of the cladding, sothat light in the core impinging the boundary with the cladding at acritical angle is confined in the core by total internal reflection.

As stated, total internal reflection occurs only for light raysimpinging the internal surface of the optical fiber with an angle whichis larger than the critical angle. Thus, a calculation performedaccording to geometrical optics may provide the largest angle which isallowed for total internal reflection to take place. An importantparameter of every optical fiber (or any other light transmittingoptical system) is known as the “numerical aperture,” which is definedas the sine of the largest incident light ray angle that is successfullytransmitted through the optical fiber, multiplied by the index ofrefraction of the medium from which the light ray enters the opticalfiber.

Another optical system designed for guiding light is the graded-indexoptical fiber, in which the light ray is guided by refraction ratherthan by total internal reflection. In this optical fiber, the refractiveindex decreases gradually from the center outwards along the radialdirection, and finally drops to the same value as the cladding at theedge of the core. As the refractive index does not change abruptly atthe boundary between the core and the cladding, there is no totalinternal reflection. However, although no total internal reflectiontakes place, the refraction bends the guided light rays back into thecenter of the core while the light passes through layers with lowerrefractive indexes.

Optical fibers are available in various lengths and core-diameters. Forlarge core diameters, glass optical fibers are known to be more brittleand fragile than plastic optical fiber.

Another type of optical system is based on photonic materials, where thelight ray is confined within a band gap material surrounding the lightray. In this type of optical system, also known as a photonic materialwaveguide, the light is confined in the vicinity of low-index region.One example of a photonic material waveguide is a silica fiber having anarray of small air holes throughout its length. This configuration iscapable of providing lossless light transmitting, e.g., in eithercylindrical or planar type waveguides.

The above description holds both for polarized and unpolarized light.When polarized light is used, an additional electromagnetic phenomenoninfluences the reflection of the light, as further explainedhereinbelow.

Polarized light is produced when the direction of the electromagneticfields in the plane perpendicular to the direction of propagation areconstrained in some fashion. For the purpose of providing a simpleexplanation, only the electric field is discussed herein. Acomplementary explanation, regarding the magnetic field, can be easilyobtained by one ordinarily skilled in the art by considering themagnetic field as being perpendicular to both the direction ofpropagation and the electric field.

The light is said to be elliptically polarized when two perpendicularcomponents of the electric field have a constant phase difference, andthe tip of the electric field vector traces out an ellipse in the planeperpendicular to the direction of propagation. Linearly polarized lightis a special case of elliptically polarized light, where the twocomponents oscillate in phase and the electric vector traces out astraight line.

Circularly polarized light is also a special case of ellipticallypolarized light in which the two components have a 90° phase differenceand the electric field vector traces out a circle in the planeperpendicular to the direction of propagation. When viewed lookingtowards the source, a right circularly polarized beam at a fixedposition as a function of time has a field vector that describes aclockwise circle, while left circularly polarized light has a fieldvector that describes a counter-clockwise circle.

When a polarized light strike a surface between two different materials,it is useful to define the polarization of the light relative to thesurface, typically horizontal and vertical polarizations, with respectto the surface. When the light strikes a material having associatedvalues of permeability, permittivity and conductivity, a portion of theenergy carried by the light is lost due non-ideal conductivity of thematerial. The relative portion of the energy which is lost is defined asthe reflection coefficient of the material. The reflective coefficientvaries according to the angle of incidence, the polarization of theincoming wave, its frequency and the characteristics of the surface. Forhorizontal polarizations the coefficient may be generalized to aconstant value, whereas for vertical polarizations however, thecoefficient varies between 0 and 1.

When the reflective coefficient value goes to zero, the light is notreflected from the surface. This phenomenon is known as the Brewstereffect, and the angle at which there is not reflection (for a particularpolarization) is called the Brewster angle. This angle often referred toas the polarizing angle, since an unpolarized wave incident on aninterface at this angle is reflected as a polarized wave with itselectric vector being perpendicular to the plane of incidence.

The present embodiments successfully provide illumination apparatuswhich provides surface illumination at any brightness, intensity andcolor profile. As further detailed hereinunder, there is an additionalphysical phenomenon which may be exploited by the illumination apparatusof the present embodiments. This phenomenon is known as lightscattering.

Unlike the above mention reflection, where radiation is deflected fromthe surface in one direction, some particles and molecules, also knownas scatterers, have the ability to scatter radiation in more than onedirection. Many types of scatterers are known. Broadly speaking,scatterers may be categorized into two groups: (i) selective scatterers,which are more effective at scattering a particular wavelength (i.e.,color), or a narrow range of wavelengths, of the light; and (ii)non-selective scatterers are capable of scattering light in a wide rangeof wavelengths.

Referring now to the drawings, FIGS. 1 a-b illustrate a perspective view(FIG. 1 a) and a section view along line A-A (FIG. 1 b) of anillumination apparatus 10, according to various exemplary embodiments ofthe present invention.

Apparatus 10 comprises one or more light emitting sources 12 embedded ina waveguide material 14 having a first surface 16 and a second surface18. Waveguide material 14 is capable of propagating light generated bylight source 12, such that at least a portion of the light is diffusedwithin waveguide material 14 and exits through at least a portion offirst surface 16.

The terms “light source” or “light emitting source”, are used hereininterchangeably and refer to any self light emitting element, including,without limitation, an inorganic light emitting diode, an organic lightemitting diode or any other electroluminescent element. The term “lightsource” as used herein refers to one or more light sources.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

Organic light emitting diodes suitable for the present embodiments canbe bottom emitting OLEDs, top emitting OLEDs and side emitting OLEDs,having one or two transparent electrodes.

As used herein, “top” refers to furthest away from surface 18, while“bottom” refers to closest to surface 18.

The waveguide material according to embodiments of the present inventionmay be similar to, and/or be based on, the teachings of U.S. patentapplication Ser. Nos. 11/157,190, 60/580,705 and 60/687,865, allassigned to the common assignee of the present invention and fullyincorporated herein by reference. Alternatively, the waveguide materialaccording to some embodiments of the present invention may also haveother configurations and/or other methods of operation as furtherdetailed hereinunder.

Waveguide material 14 can be translucent or clear as desired. In anyevent, since waveguide material 14 propagates and emits the lightemitted by light source 12, it is transparent at least to thecharacteristic emission spectrum of light source 12. The characteristicemission spectrum of the light source is also referred to herein as “thecolor” of the light source. Thus, for example, a light emitting sourcecharacterized by a spectrum having an apex at a wavelength of from about420 to about 500 nm, is referred to as a “blue light source”, a lightemitting source characterized by a spectrum having an apex at awavelength of from about 520 to about 580 nm, is referred to as a “greenlight source”, a light emitting source characterized by a spectrumhaving an apex at a wavelength of about 620-680 nm, is referred to as a“red light source”, and so on for other colors. This terminology iswell-known to those skilled in the art of optics.

As used herein the term “about” refers to ±10%.

Waveguide material 14 is optionally and preferably flexible, and mayalso have a certain degree of elasticity. Thus, material 14 can be, forexample, an elastomer. It is to be understood that although waveguidematerial appears to be flat in FIGS. 1 a-b, this need not necessarily bethe case since for some applications it may not be necessary for theillumination apparatus to be flat. FIG. 1 c schematically illustrates aperspective view of apparatus 10 in a preferred embodiment in whichwaveguide material 14 is non-planar. Further, although apparatus 10 isshown as opaque from one direction, this is only for the clarity ofpresentation and need not necessarily be the case; the surfaces ofapparatus 10 are not necessarily opaque.

According to a preferred embodiment of the present invention apparatus10 comprises a reflecting surface 32 which prevents emission of lightthrough surface 18 and therefore enhances emission of light throughsurface 16. Surface 32 can be made of any light reflecting material, andcan be either embedded in or attached to waveguide material 14.Apparatus 10 can further comprise a printed circuit board (not shown,see reference numeral 26 in FIG. 2 b) which supplies the forward bias tothe embedded light source.

There are several advantages for embedding the light source within thewaveguide material. One advantage is that all the light emitted from thelight source eventually arrives at the waveguide material. When thelight source is externally coupled to the waveguide material, some ofthe light scatters at wide angle and does not impinge the waveguidematerial. Thus, the embedding of light source 12 in waveguide material14 allows to efficiently collect the emitted light.

Another advantage is the optical coupling between the light source andthe waveguide material in particular when the light source is a lightemitting diode. When the diode is externally coupled to the waveguidematerial, the light emitted from the p-n junction should be transmittedout of the diode into the air, and subsequently from the air into thewaveguide material. The mismatch of impedances in each such transitionsignificantly reduces the coupling efficiency due to unavoidablereflections when the light passes from one medium to the other. On theother hand, when the diode is embedded in waveguide material, there is adirect transition of light from the diode to the waveguide material withhigher overall transmission coefficient. To further improve the couplingefficiency, the waveguide material is preferably selected with arefraction index which is close to the refraction index of the diode.Typical difference in refraction indices is from about 1.5 to about 1.6.

Light source 12 can be a LED, which includes the bare die and all theadditional components packed in the LED package, or, more preferably,light source 12 can include the bare die, excluding one or more of theother components (e.g., reflecting cup, silicon, LED package and thelike).

As used herein “bare die” refers to a p-n junction of a semiconductormaterial. When a forward biased is applied to the p-n junction throughelectrical contacts connected to the p side and the n side of the p-njunction, the p-n junction emits light at a characteristic spectrum.

Thus, in various exemplary embodiments of the invention light source 12includes only the semiconductor p-n junction and the electricalcontacts. Also contemplated are configurations in which several lightsources are LEDs, and several light sources other are bare dies withelectrical contacts connected thereto.

The advantage of using a bare die rather than a LED is that some of thecomponents in the LED package including the LED package absorb part ofthe light emitted from the p-n junction and therefore reduce the lightyield.

Another advantage is that the use of bare die reduces the amount of heatgenerated during light emission. This is because heat is generated dueto absorption of light by the LED package and reflecting cup. Theconsequent increase in temperature of the p-n junction causes thermalimbalance which is known to reduce the light yield. Since the bare diedoes not include the LED package and reflecting cup, the embedding of abare die in the waveguide material reduces the overall amount of heatand increases the light yield. The elimination of the LED packagepermits the use of many small bare dies instead of each large packagedLED. Such configuration allows operating each bare die in low powerwhile still producing sufficient overall amount of light, thereby toimproving the p-n junction efficacy.

An additional advantage is light diffusion within the waveguidematerial. The minimization of redundant components in the vicinity ofthe p-n junction results in almost isotropic emission of light from thep-n junction which improves the diffusion of light. To further improvethe coupling efficiency, the waveguide material is preferably selectedwith a refraction index which is close to the refraction index of thep-n junction.

Light source 12 can be embedded in the bulk of waveguide material 14 ornear surface 18. FIG. 2 a is a fragmentary view schematicallyillustrating the embodiment in which light source 12 is embedded in thebulk of material 14 and FIG. 2 b is fragmentary view schematicallyillustrating the embodiment in which light source 12 is embedded nearsurface 18. It is to be understood that FIGS. 2 a-b illustrate a singlelight source 12 for clarity of presentation and it is not intended tolimit the scope of the present invention to such configuration. Asstated, apparatus 10 can comprise one or more light emitting sources.

Referring to FIG. 2 a, when light source 12 is embedded in the bulk ofthe waveguide material, the electrical contacts 20 remain with material14. In this embodiment, the forward bias can be supplied to light source12 by electrical lines 22, such as flexible conductive wires, which arealso embedded in material 14. Thus, lines 22 extend from contacts 20 toone or more ends 28 of waveguide material 14. Light source 12 includingthe electrical lines 22 can be embedded in material 14 during themanufacturing process of the waveguide material. When a plurality oflight sources are embedded in the waveguide material, they can beconnected to an arrangement of electrical lines as known in the art andthe entire of light sources and arrangement of electrical lines can beembedded in the material during the manufacturing process of thewaveguide material.

In various exemplary embodiments of the invention light source 12 isoperated with low power and therefore does not produce large amount ofheat. This is due to the relatively large light yield of the embeddedlight source and the high optical coupling efficiency between the lightsource and the waveguide material. In particular, when light source 12is a bare die, its light yield is significantly high while the producedheat is relatively low. Light source 12 can also be operated usingpulsed electrical current which further reduces the amount of producedheat.

Preferably, but not obligatorily, light source 12 is encapsulated by atransparent thermal isolating encapsulation 24. Encapsulation 24 servesfor thermally isolating the light source from material 14. Thisembodiment is particularly useful when light source 12 is a bare die, inwhich case the bare die radiate heat which may change the opticalproperties of material 14. Alternatively or additionally, waveguidematerial 14 can be made with high specific heat capacity to allowmaterial 14 to receive heat from light source 12 with minimal or noundesired heating effects.

Referring to FIG. 2 b, when light source 12 is embedded near surface 18of material 14, electrical contacts 20 can remain outside material 14 atsurface 18 and can therefore be accessed without embedding electricallines in material 14. The electrical contacts can be applied withforward bias using external electrical lines or directly from a printedcircuit board 26. In this embodiment, board 26 can be made, at least inpart, of heat conducting material so as to facilitate evacuation of heataway from light source 12. When the heat evacuation by board 26 issufficient, light source 12 can be embedded without thermal isolatingencapsulation 24.

As stated, waveguide material 14 is capable of propagating and diffusingthe light until it exits though surface 16 or a portion thereof. It willbe appreciated that this ability of the waveguide material, combinedwith the high light yield and efficient optical coupling between theembedded light sources and the waveguide material, provide apparatus 10with properties suitable for many applications.

As is further detailed hereinunder, there are many alternatives toconstruct the waveguide material, which provide flexibility in itsdesign. In particular the waveguide material can be tailored accordingto the desired optical properties of the waveguide. Thus, thedistribution of light sources within the waveguide material and/or theoptical properties of the waveguide material can be selected to providethe most suitable illumination for the specific application for whichapparatus 10 is used. More specifically apparatus 10 can provideillumination at a predetermined light profile, which is manifested by apredetermined intensity profile, predetermined brightness profile andpredetermined color profile. Such illumination apparatus can thereforeprovide high light quality in terms of brightness, intensity and/orcolor profiles.

For example, light sources emitting different colors of light (i.e.,light sources having different characteristic emission spectra, whichmay or may not have spectral overlaps therebetween), say two, three ormore different colors, can be distributed in the waveguide such thatsurface 16 emits light at a predetermined light profile. Additionally,the optical properties of the waveguide material can be made local andwavelength-dependent according to the predetermined light profile. Morespecifically, according to the presently preferred embodiment of theinvention, different regions in the waveguide material have a differentresponse to different light spectra.

In various exemplary embodiments of the invention apparatus 10 comprisesone or more photoluminescent layers 30 coating surface 16 or a portionthereof. Photo luminescent layer 30 comprises a photoluminescentmaterial which can be a phosphor or a fluorophore.

The term “photoluminescent layer” is commonly used herein to describeone photoluminescent layer or a plurality of photoluminescent layers.Additionally, a photoluminescent layer can comprise one or more types ofphotoluminescent molecules. In any event, a photoluminescent layer ischaracterized by an absorption spectrum (i.e., a range of wavelengths oflight which can be absorbed by the photoluminescent molecules to effectquantum transition to a higher energy level) and an emission spectrum arange of wavelengths of light which are emitted by the photoluminescentmolecules as a result of quantum transition to a lower energy level).The emission spectrum of the photoluminescent layer is typically widerand shifted relative to its absorption spectrum. The difference inwavelength between the apex of the absorption and emission spectra ofthe photoluminescent layer is referred to as the Stokes shift of thephotoluminescent layer.

The absorption spectrum of photoluminescent layer 30 preferably overlapsthe emission spectrum of at least one of light sources 12. Morepreferably, for each characteristic emission spectrum of an embeddedlight source, there is at least one photoluminescent layer having anabsorption spectrum overlapping the emission spectrum the light source.According to a preferred embodiment of the present invention the apex ofthe light source's emission spectrum lies in the spectrum of thephotoluminescent layer, and/or the apex of the photoluminescent layer'sabsorption spectrum lies in the spectrum of the light source.

Photoluminescent layer 30 serves for “converting” the wavelength of aportion of the light emitted by light sources 12. More specifically, foreach photon which is successfully absorbed by layer 30, a new photon isemitted. Depending on the type of photoluminescent, the emitted photoncan have a wavelength which is longer or shorter than the wavelength ofthe absorbed photon. Photons which do not interact with layer 30propagate therethrough. The combination of converted light andnon-converted light forms the light profile of apparatus 10.

Phosphors are widely used for coating individual LEDs, typically in thewhite LEDs industry. However, photoluminescent layers covering anilluminating surface of a waveguide material such as the waveguidematerial of the present embodiments have not been employed. Theadvantage of using layer 30 over waveguide material 14, as opposed to oneach individual light emitting source 12, is that waveguide material 14first diffuses the light and thereafter emits it through surface 16.Thus, instead of collecting light from a point light source (e.g., aLED), layer 30 collects light from a surface light source having apredetermined area (surface 16 or a portion thereof). This configurationallows a better control on the light profile provided by apparatus 10.

Many types of phosphorescent and fluorescent substance are contemplated.Representative examples include, without limitation the phosphorsdisclosed in U.S. Pat. Nos. 5,813,752, 5,813,753, 5,847,507, 5,959,316,6,155,699, 6,351,069, 6,501,100, 6,501,102, 6,522,065, 6,614,179,6,621,211, 6,635,363, 6,635,987, 6,680,004, 6,765,237, 6,853,131,6,890,234, 6,917,057, 6,939,481, 6,982,522, 7,015,510, 7,026,756 and7,045,826 and 7,005,086.

The various possible light profile options make the apparatus of thepresent embodiments suitable for providing illumination in manyapplications. Representative examples of uses of apparatus 10,including, without limitation, architectural highlighting, decorativelighting, medical lighting, signage for displaying commercial ordecorative expressions, visual guidance (e.g., landing strips, aisles),display, exhibit lighting, roadway lighting, automotive lighting and thelike. The flexibility of the waveguide material makes apparatus 10attachable to many surfaces, including, without limitation, walls of abuilding (either external or internal), windows, boxes (e.g., jewelryboxes), toys and the like.

Although apparatus 10 can be designed to provide any light profile, formany applications it is desired to construct apparatus 10 to providesubstantially uniform illumination. The apparatus of the presentembodiments can provide illumination characterized by a uniformity of atleast 70%, more preferably at least 80%, even more preferably at least90%. This is particularly useful when apparatus 10 is incorporated in abacklight unit of a passive display device.

Whit light illumination can be provided in more than one way. In oneembodiment, the waveguide material is embedded with red light sources,green light sources, blue light sources and optionally light sources ofother colors (e.g., orange, yellow, green-yellow, cyan, amber,blue-violet) which are distributed such that the combination of redlight, green light, blue light and optionally light in the other colorsappears as a substantially uniform white light across the area ofsurface 16 or a portion thereof.

In another embodiment, layer 30 serves for complementing the lightemitted by light sources 12 to a white light, e.g., using dichromatic,trichromatic, tetrachromatic or multichromatic approach.

For example, a blue-yellow dichromatic approach can be employed, inwhich case blue light sources (e.g., bare dies of InGaN with a peakemission wavelength at about 460 nm), can be distributed in waveguidematerial 14, and layer 30 can be made of phosphor molecules withabsorption spectrum in the blue range and emission spectrum extending tothe yellow range (e.g., cerium activated yttrium aluminum garnet, orstrontium silicate europium). Since the scattering angle of lightsharply depends on the frequency of the light (fourth power dependencefor Rayleigh scattering, or second power dependence for Mie scattering),the blue light generated by the blue light sources is efficientlydiffused in the waveguide material and exits, substantially uniformly,through surface 16 into layer 30. Layer 30 which has no preferreddirectionality, emits light in its emission spectrum and complements theblue light which is not absorbed to white light.

In another dichromatic configurations, ultraviolet light sources (e.g.,bare dies of GaN, AlGaN and/or InGaN with a peak emission wavelengthbetween 360 nm and 420 nm), can be distributed in waveguide material 14.Light of such ultraviolet light sources is efficiently diffused in thewaveguide material and exits, substantially uniformly, through surface16. To provide substantially white light, two photoluminescent layersare preferably deposited on surface 16. One layer can be characterizedby an absorption spectrum in the ultraviolet range and emission spectrumin the orange range (with peak emission wavelength from about 570 nm toabout 620 nm), and another layer characterized by an absorption spectrumin the ultraviolet range and emission spectrum in the blue-green range(with peak emission wavelength from about 480 nm to about 500 nm). Theorange light and blue-green light emitted by the two photoluminescentlayers blend to appear as white light to a human observer. Since thelight emitted by the ultraviolet light sources is above or close to theend of visual range it is not seen by the human observer. The twophotoluminescent layers are preferably deposited one on top of the othersuch as to improve the uniformity. Alternatively, a single layer havingtwo types of photoluminescent with the above emission spectra can bedeposited.

In another embodiment a trichromatic approach is employed. For example,blue light sources can be distributed in the waveguide material asdescribed above, with two photoluminescent layers deposited on surface16. A first photoluminescent layer can be made of phosphor moleculeswith absorption spectrum in the blue range and emission spectrumextending to the yellow range as described above, and a secondphotoluminescent layer absorption spectrum in the blue range andemission spectrum extending to the red range (e.g., cerium activatedyttrium aluminum garnet doped with a trivalent ion of praseodymium, oreuropium activated strontium sulphide). The unabsorbed blue light, theyellow light and the red light blend to appear as white light to a humanobserver.

Also contemplated is a configuration is which light sources withdifferent emission spectra are distributed and several photoluminescentlayers are deposited, such that the absorption spectrum of eachphotoluminescent layer overlaps one of the emission spectra of the lightsources, and all the emitted colors (of the light sources and thephotoluminescent layers) blend to appear as white light. The advantageof such multi-chromatic configuration is that it provides high qualitywhite balance because it allows better control on the various spectralcomponents of the light in a local manner across the surface of theillumination apparatus.

The color composite of the white output light depends on the intensitiesand spectral distributions of the emanating light emissions. Thesedepend on the spectral characteristics and spatial distribution of thelight sources, and, in the embodiments in which one or morephotoluminescent layers are employed, on the spectral characteristics ofthe photoluminescent layer(s) and the amount of unabsorbed light. Theamount of light that is unabsorbed by the photoluminescent layer(s) isin turn a function of the thickness of the photoluminescent layer(s),the density of photoluminescent material(s) and the like. By judiciouslyselecting the emission spectra of light emitting source 12 andoptionally the thickness, density, and spectral characteristics(absorption and emission spectra) of layer 30, apparatus 10 can be madeto serve as an illumination surface (either planar or non planar, eitherstiff of flexible) which provides substantially uniform white light.

In any of the above embodiments, the “whiteness” of the light can betailored according to the specific application for which apparatus 10 isused. For example, when apparatus 10 is incorporated for backlight of anLCD device, the spectral components of the light provided by apparatus10 can be selected in accordance with the spectral characteristics ofthe color filters of the liquid crystal panel. In other words, since atypical liquid crystal panel comprises an arrangement of color filtersoperating at a plurality of distinct colors, the white light provided byapparatus 10 includes at least at the distinct colors of the filters.This configuration significantly improves the optical efficiency as wellis the image quality provided by the LCD device, because the opticallosses due to mismatch between the spectral components of the backlightunit and the color filters of the liquid crystal panel are reduced oreliminated.

Thus, in the embodiment in which the white light is achieved by lightsources emitting different colors of light (e.g., red light, green lightand blue light), the emission spectra of the light sources arepreferably selected to substantially overlap the characteristic spectraof the color filters of the LCD panel. In the embodiment in whichapparatus 10 is supplemented by one or more photoluminescent layers theemission spectra of the photoluminescent layers and optionally theemission spectrum or spectra of the light sources are preferablyselected to overlap the characteristic spectra of the color filters ofthe LCD panel. Typically the overlap between a characteristic emissionspectrum and a characteristic filter spectrum is about 70% spectraloverlap, more preferably about 80% spectral overlap, even morepreferably about 90%.

Reference is now made to FIG. 3 which is a section view along line A-Aof FIG. 1 a, according to a preferred embodiment in which apparatus 10comprises a structured film 34. Structured film 34 can be, for example,a brightness enhancement film and it can be deposited on or embedded inwaveguide material 14. Film 34 collimates the light emitted from lightsources 12 thereby increasing the brightness of the illuminationprovided by apparatus 10. This embodiment is particularly useful whenapparatus 10 is used for backlight of an LCD device. The increasedbrightness enables a sharper image to be produced by the liquid crystalpanel and allows operating the light sources at low power to produce aselected brightness. The structured film can operate according toprinciples and operation of prisms. Thus, light rays arriving thestructured film at small angles relative to the normal to the structuredfilm are reflected, while other light rays are refracted. The reflectedlight rays continue to propagate and diffuse in the waveguide materialuntil they arrive at the structured film at a sufficiently large angle.In the embodiment in which apparatus 10 comprises the reflecting surface32 it prevents the light which is reflected from film 34 to exit throughsurface 18. Structured films are known in the art and are found in theliterature, see, e.g., International Patent Application Publication No.WO 96/023649.

Reference is now made to FIG. 4 which is a fragmentary viewschematically illustrating an embodiment in which apparatus 10 comprisesone or more optical elements 36 embedded waveguide material 14 forenhancing the diffusion of light. One skilled in the art will recognizethat several components of apparatus 10 have been omitted from FIG. 4for clarity of presentation. Element 36 can be embedded in material 14near surface 16 or at any other location.

In various exemplary embodiments of the invention element 36 operates asan angle-selective light transmissive element. Specifically, element 36is preferably configured to reflect light striking element 36 at apredetermined range of angles (say, ±10° from the normal to surface 16),and transmit light striking element 36 at other angles. Element 36 canbe a mini prism, a structured surface similar to surface 34 above, amicrolens and the like. Element 36 can be embedded in material 14 duringthe manufacturing process of material 14 in parallel to the embedding oflight source 12 or any other component. The size of element 36 isselected to allow the collection of light rays at the predeterminedrange of angles and therefore depends on the distance between surface 16and light source 12. Thus, in the embodiments in which light source 12is embedded near surface 18, element 36 has larger size compared to itssize in the embodiments in which light source 12 is embedded in the bulkof material 14.

Reference is now made to FIG. 5 which is a block diagram schematicallyillustrating a liquid crystal display device 40, according to variousexemplary embodiments of the present invention. Device 40 comprises aliquid crystal panel 42 and a backlight unit 44. Backlight unit 44 canbe or it can comprise comprises illumination apparatus 10 as furtherdetailed hereinabove. Several components of apparatus 10 have beenomitted from FIG. 5 for clarity of presentation, but one of ordinaryskill in the art, provided with the details described herein would knowhow to construct apparatus 10 according to the various exemplaryembodiments desribed above.

Panel 42 comprises a matrix of thin film transistors 46 fabricated on asubstrate 48 of glass or another transparent material. A liquid crystalfilm 50 is disposed over substrate 48 and transistors 46. A polarizer 56is disposed on a backside of substrate 48. Transistors 16 can beaddressed by gate lines (not shown) deposited on the substrate 48 duringthe fabrication of transistors 16 as is well known in the art. Eachparticular transistor applied by voltage conducts electrical current andcharges film 50 in its vicinity. The charging of the liquid crystal filmalters the opacity of the film, and effects a local change in lighttransmission of the liquid crystal film 20. Hence, transistors 16 definedisplay cells 52 (e.g., pixels) in liquid crystal film 50. Typically,the opacity of each display cells is charged to one of several discreteopacity levels to implement an intensity gray scale. Thus, the displaycells serves as a gray scale picture elements. However, pixel opacityalso can be controlled in a continuous analog fashion or a digital(on/oft) fashion.

Color-selective filters 54 are distributed on cells 52 across thedisplay area of panel 42 to produce a color display. Typically, but notobligatorily, there are three types of color filters (designated in FIG.5 by 54 a, 54 b and 54 c) where each filter allows transmission of oneof the three primary additive colors red, green and blue. The schematicblock diagram of FIG. 5 illustrates a single three-component cell whichincludes a first component color (e.g., red output by cell 52 covered byfilter 54 a), a second component color (e.g., green output by cell 52covered by filter 54 b), and a third component color (e.g., blue outputby the cell 52 covered by the filter 54 c) that are selectively combinedor blended to generate a selected color.

In operation, backlight unit 44 produces a substantially uniform whiteillumination as detailed above, and polarizer 56 optimizes the lightpolarization with respect to polarization properties of liquid crystalfilm 20. The opacity of the cells 52 is modulated using transistors 46as detailed above to create a transmitted light intensity modulationacross the area of device 40. Color filters 54 colorize theintensity-modulated light emitted by the pixels to produce a coloroutput. By selective opacity modulation of neighboring display cells 52of the three color components, selected intensities of the three colorsare blended together to selectively control color light output. As isknown in the art, selective blending of three primary colors such asred, green, and blue can generally produce a full range of colorssuitable for color display purposes. Spatial dithering is optionally andpreferably used to provide further color blending across neighboringcolor pixels

Display device 40 can be incorporated in may applications.Representative examples include, without limitation, a portable computersystem (e.g., a laptop), a computer monitor, a personal digitalassistant system, a cellular communication system (e.g., a mobiletelephone), a portable navigation system, a television system and thelike.

Additional objects, advantages and features of the present embodimentswill become apparent to one ordinarily skilled in the art uponexamination of the following examples for constructing waveguidematerial 14, which are not intended to be limiting. According to apreferred embodiment of the present invention waveguide material 14comprises a polymeric material. The polymeric material may optionallycomprise a rubbery or rubber-like material. According to a preferredembodiment of the present invention material 14 is formed by dip-moldingin a dipping medium, for example, a hydrocarbon solvent in which arubbery material is dissolved or dispersed. The polymeric materialoptionally and preferably has a predetermined level of cross-linking,which is preferably between particular limits. The cross-linking mayoptionally be physical cross-linking, chemical cross-linking, or acombination thereof. A non-limiting illustrative example of a chemicallycross-linked polymer comprises cross-linked polyisoprene rubber. Anon-limiting illustrative example of a physically cross-linked polymercomprises cross-linked comprises block co-polymers or segmentedco-polymers, which may be cross-linked due to micro-phase separation forexample. Material 14 is optionally cross-linked through application of aradiation, such as, but not limited to, electron beam radiation andelectromagnetic radiation.

Although not limited to rubber itself, the material optionally andpreferably has the physical characteristics of rubber, such asparameters relating to tensile strength and elasticity, which are wellknown in the art. For example, material 14 is preferably characterizedby a tensile set value which is below 5%. The tensile set valuegenerally depends on the degree of cross-linking and is a measure of theability of flexible material 14, after having been stretched either byinflation or by an externally applied force, to return to its originaldimensions upon deflation or removal of the applied force.

The tensile set value can be determined, for example, by placing tworeference marks on a strip of material 14 and noting the distancebetween them along the strip, stretching the strip to a certain degree,for example, by increasing its elongation to 90% of its expectedultimate elongation, holding the stretch for a certain period of time,e.g., one minute, then releasing the strip and allowing it to return toits relaxed length, and re-measuring the distance between the tworeference marks. The tensile set value is then determined by comparingthe measurements before and after the stretch, subtracting one from theother, and dividing the difference by the measurement taken before thestretch. In a preferred embodiment, using a stretch of 90% of itsexpected ultimate elongation and a holding time of one minute, thepreferred tensile set value is less than 5%. Also contemplated arematerials having about 30% plastic elongation and less then 5% elasticelongation.

The propagation and diffusion of light through material 14 can be donein any way known in the art, such as, but not limited to, total internalreflection, graded refractive index and band gap optics. Additionally,polarized light may be used, in which case the propagation of the lightcan be facilitated by virtue of the reflective coefficient of material14. For example, a portion of material 14 can be made of a dielectricmaterial having a sufficient reflective coefficient, so as to trap thelight within at least a predetermined region.

In any event, material 14 is preferably designed and constructed suchthat at least a portion of the light propagates therethrough at aplurality of directions, so as to allow the diffusion of the light inmaterial 14 and the emission of the light through more than one point insurface 16.

Reference is now made to FIGS. 6 a-b, which illustrates material 14 inan embodiment in which total internal reflection is employed. In thisembodiment material 14 comprises a first layer 62 and a second layer 64.Preferably, the refractive index of layer 66, designated in FIGS. 6 a-bby n₁, is smaller than the refractive index, n₂, of layer 64. In suchconfiguration, when the light, shown generally at 58, impinges oninternal surface 65 of layer 64 at an impinging angle, θ, which islarger than the critical angle, θ_(c)≡ sin⁻¹(n₁/n₂), the light energy istrapped within layer 64, and the light propagates therethrough in apredetermined propagation angle, α. FIGS. 6 b-c, schematicallyillustrate embodiments in which material 14 has three layers, 62, 64 and66, where layer 64 is interposed between layer 62 and layer 66. In thisembodiment, the refractive index of layers 62 and 64 is smaller than therefractive index of layer 64. As shown, light source 12 can be embeddedin layer 64 (see FIG. 6 b) or it can be embedded in a manner such thatit extends over two layers (e.g., layers 62 and 64 see FIG. 6 c).

The light may also propagate through waveguide material 14 when theimpinging angle is smaller than the critical angle, in which case oneportion of the light is emitted and the other portion thereof continueto propagate. This is the case when material 14 comprises dielectric ormetallic materials, where the reflective coefficient depends on theimpinging angle, θ.

The propagation angle, α, is approximately ±(π/2−θ), in radians. αdepends on the ratio between the indices of refraction of the layers.Specifically, when n₂ is much larger than n₁, α is large, whereas whenthe ratio n₂/n₁ is close to, but above, unity, α is small. According toa preferred embodiment of the present invention the thickness of thelayers of material 14 and the indices of refraction are selected suchthat the light propagates in a predetermined propagation angle. Atypical thickness of each layer is from about 10 μm to about 3 mm, morepreferably from about 50 μm to about 500 μm, most preferably from about100 μm to about 200 μm. The overall thickness of material 14 depends onthe height of light source 12. For example, when light source 12 is aLED device of size 0.6 mm (including the LED package), the height ofmaterial 14 is preferably from about 0.65 mm to about 0.8 mm. When lightsource 12 is a bare die of size 0.1 mm, the height of material 14 ispreferably from about 0.15 mm to about 0.2 mm.

The difference between the indices of refraction of the layers ispreferably selected in accordance with the desired propagation angle ofthe light. According to a preferred embodiment of the present invention,the indices of refraction are selected such that propagation angle isfrom about 2 degrees to about 15 degrees. For example, layer 64 may bemade of poly(cis-isoprene), having a refractive index of about 1.52, andlayers 62 and 66 may be made of Poly(dimethyl siloxane) having arefractive index of about 1.45, so that Δn=n₂−n₁≈0.07 and n₂/n₁≈0.953corresponding to a propagation angle of about ±19 degrees.

The emission of the light from surface 16 of material 14 may be achievedin more than one way. Broadly speaking, one or more of the layers ofwaveguide material 14 preferably comprises at least one additionalcomponent 71 (not shown, see FIGS. 7 a-d) designed and configured so asto allow the emission of the light through the surface. Following areseveral examples for the implementation of component 71, which are notintended to be limiting.

Referring to FIG. 7 a, in one embodiment, component 71 is implemented asat least one impurity 70, present in second layer 64 and capable ofemitting light, so as to change the propagation angle of the light.Impurity 70 may serve as a scatterer, which, as stated, can scatterradiation in more than one direction. When the light is scattered byimpurity 70 in such a direction that the impinging angle, θ, which isbelow the aforementioned critical angle, θ_(c), no total internalreflection occurs and the scattered light is emitted through surface 16.According to a preferred embodiment of the present invention theconcentration and distribution of impurity 70 is selected such that thescattered light is emitted from a predetermined region of surface 16.More specifically, in regions of waveguide material 14 where largerportion of the propagated light is to be emitted through the surface,the concentration of impurity 70 is preferably large, while in regionswhere a small portion of the light is to be emitted the concentration ofimpurity 70 is preferably smaller.

As will be appreciated by one ordinarily skilled in the art, the energytrapped in waveguide material 14 decreases each time a light ray isemitted through surface 16. On the other hand, it is often desired touse material 14 to provide a uniform surface illumination. Thus, as theoverall amount of energy decreases with each emission, a uniform surfaceillumination can be achieved by gradually increasing the ratio betweenthe emitted light and the propagated light. According to a preferredembodiment of the present invention, the increasing emitted/propagatedratio is achieved by an appropriate selection of the distribution ofimpurity 70 in layer 64. More specifically, the concentration ofimpurity 70 is preferably an increasing function of the optical distancewhich the propagated light travels.

Optionally, impurity 70 may comprise any object that scatters light andwhich is incorporated into the material, including but not limited to,beads, air bubbles, glass beads or other ceramic particles, rubberparticles, silica particles and so forth, any of which may optionally befluorescent particles or biological particles, such as, but not limitedto, Lipids.

FIG. 7 b further details the presently preferred embodiment of theinvention. In FIG. 7 b, impurity 70 is optionally and preferablyimplemented as a plurality of particles 42, distributed in an increasingconcentration so as to provide a light gradient. Particles 42 arepreferably organized so as to cause light to be transmitted withsubstantially lowered losses through scattering of the light. Particles42 may optionally be implemented as a plurality of bubbles in a solidplastic portion, such as a tube for example. According to a preferredembodiment of the present invention the size of particles 42 is selectedso as to selectively scatter a predetermined range of wavelengths of thelight. More specifically small particles scatter small wavelengths andlarge particles scatter both small and large wavelengths.

Particles 42 may also optionally act as filters, for example forfiltering out particular wavelengths of light. Preferably, differenttypes of particles 42 are used at different locations in waveguidematerial 14. For example, particles 42 which are specific to scatteringof a particular spectrum may preferably be used within waveguidematerial 14, at locations where such particular wavelength is to beemitted from waveguide material 14 to provide illumination.

According to a preferred embodiment of the present invention impurity 70is capable of producing different optical responses to differentwavelengths of the light. The difference optical responses can berealized as different emission angles, different emission wavelengthsand the like. For example, different emission wavelengths may beachieved by implementing impurity 70 as beads each having predeterminedcombination of color-components, e.g., a predetermined combination offluorophore molecules.

When a fluorophore molecule embedded in a bead absorbs light, electronsare boosted to a higher energy shell of an unstable excited state.During the lifetime of excited state (typically 1-10 nanoseconds) thefluorochrome molecule undergoes conformational changes and is alsosubject to a multitude of possible interactions with its molecularenvironment. The energy of excited state is partially dissipated,yielding a relaxed singlet excited state from which the excitedelectrons fall back to their stable ground state, emitting light of aspecific wavelength. The emission spectrum is shifted towards a longerwavelength than its absorption spectrum. The difference in wavelengthbetween the apex of the absorption and emission spectra of afluorochrome (also referred to as the Stokes shift), is typically small.

Thus, in this embodiment, the wavelength (color) of the emitted light iscontrolled by the type(s) of fluorophore molecules embedded in thebeads. Other objects having similar or other light emission propertiesmay be also be used. Representative examples include, withoutlimitation, fluorochromes, chromogenes, quantum dots, nanocrystals,nanoprisms, nanobarcodes, scattering metallic objects, resonance lightscattering objects and solid prisms.

Referring to FIG. 7 c, in another embodiment, component 71 isimplemented as one or more diffractive optical elements 72 formed withlayer 64, for at least partially diffracting the light. Thus, thepropagated light reaches optical element 72 where a portion of the lightenergy is coupled out of material 14, while the remnant energy isredirected through an angle, which causes it to continue its propagationthrough layer 64. Optical element 70 may be realized in many ways,including, without limitation, non-smooth surfaces of layer 64, amini-prism or grating formed on internal surface 65 and/or externalsurface 67 of layer 64. Diffraction Gratings are known to allow bothredirection and transmission of light. The angle of redirection isdetermined by an appropriate choice of the period of the diffractiongrating often called “the grating function.” Furthermore, thediffraction efficiency controls the energy fraction that is transmittedat each strike of light on the grating. Hence, the diffractionefficiency may be predetermined so as to achieve an output havingpredefined light intensities; in particular, the diffraction efficiencymay vary locally for providing substantially uniform light intensities.Optical element 70 may also be selected such that the scattered lighthas a predetermined wavelength. For example, in the embodiment in whichoptical element 70 is a diffraction grating, the grating function may beselected to allow diffraction of a predetermined range of wavelengths.

Referring to FIG. 7 d, in an additional embodiment, one or more regions74 of layer 62 and/or 66 may have different indices of refraction so asto prevent the light from being reflected from internal surface 65 ofsecond layer 64. For example, denoting the index of refraction of region74 by n₃, a skilled artisan would appreciate that when n₃>n₂, no totalinternal reflection can take place, because the critical angle, θ_(c),is only defined when the ratio n₃/n₂ does not exceed the value of 1. Theadvantage of this embodiment is that the emission of the light throughsurface 16 is independent on the wavelength of the light.

As stated, flexible material 14 preferably comprises polymeric material.The polymeric material may optionally comprise natural rubber, asynthetic rubber or a combination thereof. Examples of syntheticrubbers, particularly those which are suitable for medical articles anddevices, are taught in U.S. Pat. No. 6,329,444, hereby incorporated byreference as if fully set forth herein with regard to such illustrative,non-limiting examples. The synthetic rubber in this patent is preparedfrom cis-1,4-polyisoprene, although of course other synthetic rubberscould optionally be used. Natural rubber may optionally be obtained fromHevena brasiliensis or any other suitable species.

Other exemplary materials, which may optionally be used alone or incombination with each other, or with one or more of the above rubbermaterials, include but are not limited to, crosslinked polymers such as:polyolefins, including but not limited to, polyisoprene, polybutadiene,ethylene-propylene copolymers, chlorinated olefins such aspolychloroprene (neoprene) block copolymers, including diblock-,triblock-, multiblock- or star-block-, such as:styrene-butadiene-styrene copolymers, or styrene-isoprene-styrenecopolymers (preferably with styrene content from about 1% to about 37%),segmented copolymers such as polyurethanes, polyether-urethanes,segmented polyether copolymers, silicone polymers, including copolymers,and fluorinated polymers and copolymers.

For example, optionally and preferably, the second layer comprisespolyisoprene, while the first layer optionally and preferably comprisessilicone. If a third layer is present, it also optionally and preferablycomprises silicone.

According to an optional embodiment of the present invention, theflexible material is formed by dip-molding in a dipping medium.Optionally, the dipping medium comprises a hydrocarbon solvent in whicha rubbery material is dissolved or dispersed. Also optionally, thedipping medium may comprise one or more additives selected from thegroup consisting of cure accelerators, sensitizers, activators,emulsifying agents, cross-linking agents, plasticizers, antioxidants andreinforcing agents.

It is expected that during the life of this patent many relevant waveguiding techniques will be developed and the scope of the term waveguidematerial is intended to include all such new technologies a priori.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

What is claimed is:
 1. An illumination apparatus comprising: asubstantially planar waveguide; and embedded within the waveguide, (i)at least one light source, and (ii) at least one photoluminescentmaterial for converting a wavelength of a portion of the light emittedby the at least one light source, unconverted light from the at leastone light source mixing with light converted by the at least onephotoluminescent material to produce substantially white light, whereinthe waveguide (i) retains therein substantially all light emitted by theat least one embedded light source, and (ii) causes light to propagateand diffuse within the waveguide until light is caused to exit from anexit region of a surface thereof.
 2. The illumination apparatus of claim1, wherein each said at least one light source is a bare die LED.
 3. Theillumination apparatus of claim 1, wherein each said at least one lightsource is an organic LED.
 4. The illumination apparatus of claim 1,further comprising an optical element for redirecting at least a portionof the light emitted by the at least one light source out of the exitregion.
 5. The illumination apparatus of claim 4, wherein the opticalelement comprises at least one of a plurality of impurities, one or moreprisms, one or more gratings, one or more microlenses, or a non-smoothsurface.
 6. The illumination apparatus of claim 4, wherein the opticalelement is spaced away from the at least one light source toward theexit region.
 7. The illumination apparatus of claim 1, wherein the exitregion comprises a portion of a top surface of the waveguide displacedfrom the at least one light source.
 8. The illumination apparatus ofclaim 7, wherein the at least one light source is embedded proximate thebottom surface of the waveguide.
 9. The illumination apparatus of claim1, wherein the at least one light source is embedded proximate thebottom surface of the waveguide.
 10. The illumination apparatus of claim1, further comprising an encapsulation material disposed between the atleast one light source and the waveguide.
 11. The illumination apparatusof claim 1, further comprising a reflective material proximate a bottomsurface of the waveguide.
 12. The illumination apparatus of claim 1,wherein at least one said light source is an LED incorporating at leastone said photoluminescent material.
 13. The illumination apparatus ofclaim 12, further comprising an optical element for redirecting at leasta portion of the light emitted by the at least one light source out ofthe exit region.
 14. The illumination apparatus of claim 13, wherein theoptical element comprises at least one of a plurality of impurities, oneor more prisms, one or more gratings, one or more microlenses, or anon-smooth surface.
 15. The illumination apparatus of claim 13, whereinthe optical element is spaced away from the at least one light sourcetoward the exit region.
 16. The illumination apparatus of claim 12,wherein the exit region comprises a portion of a top surface of thewaveguide displaced from the at least one light source.
 17. Theillumination apparatus of claim 16, wherein the at least one lightsource is embedded proximate the bottom surface of the waveguide. 18.The illumination apparatus of claim 12, wherein the at least one lightsource is embedded proximate the bottom surface of the waveguide. 19.The illumination apparatus of claim 12, further comprising anencapsulation material disposed between the at least one light sourceand the waveguide.
 20. An illumination apparatus comprising: asubstantially planar waveguide; and embedded within the waveguide, (i)at least one light source, and (ii) at least one photoluminescentmaterial for converting a wavelength of a portion of the light emittedby the at least one light source, unconverted light from the at leastone light source mixing with light converted by the at least onephotoluminescent material to produce substantially white light, whereinthe waveguide (i) has a planar top surface above the at least oneembedded light source, (ii) retains therein substantially all lightemitted by the at least one embedded light source, and (iii) causeslight to propagate and diffuse within the waveguide until light iscaused to exit from an exit region of a surface thereof.
 21. Theillumination apparatus of claim 20, wherein each said at least one lightsource is a bare die LED.
 22. The illumination apparatus of claim 20,wherein at least one said light source is an LED incorporating at leastone said photoluminescent material.